1. Field of the Invention
The present invention relates to porous hollow fiber membranes for use in filtration of untreated water and a method of making such porous hollow fiber membranes.
2. Description of the Prior Art
In recent years, a filtering technique utilizing filtering membranes having a permselectivity has made a remarkable progress. Those filtering membranes are currently utilized in practice in numerous applications including, for example, production of ultrapure water, preparation of medicines, sterilization and finalization of brewages and purification of drinking water. The use of the filtering membranes has widely circulated to meet with the requirement to refine water (a high degree treatment), improvement in sanitation and improvement in precision. Of the various filtering membranes, the hollow fiber membrane has a feature in that the space for installation per unitary membrane area can be reduced. However, in terms of the system of filtering operation, sand filtration is still the mainstream. For example, an overwhelming majority of service water supply plants make use of a combination of a flocculating pool and a sand filtering pool.
On the other hand, as far as the application of the filtering technique in service water supply is concerned, subsequent to the mass crisis of Cryptosporidium syndromes that broke in 1996 in a town in Saitama Prefecture, Japan, the issue of “distasteful service water” resulting from degradation of the quality of headwater brought about pressing demands for improvement in sanitation and quality of the service water in metropolis. The existing waterworks facilities equipped with the flocculating pool in combination with a sand filtering pool have now come to be unreliable in terms of sanitation.
In contrast thereto, with a filtering technique using the separation membrane, it is possible to separate a substance to be filtered with a precision that a particle cutoff is only 1/100 of that exhibited by a sand filtering and, therefore, the filtering technique using the separation membrane is highly reliable. For this reason, mainly in a small water supply system, it is a recent trend to shift from the use of the standard sand filtering equipment to the use of a membrane filtering equipment and, as a new water purifying system, the use of the membrane filtering technique is currently wide-spreading.
In the course thereof, a reason that the system in which the separation membrane is used has not yet been wide-spreading in the field of the service water supply system is because, while with the sand filtering system a flow at which the water is filtered per unitary filtering area is considerably high (for example, 3 to 10 m3/m2/d at a slow filtration or 120 to 1,500 m3/m2/d at a rapid filtration), resulting in low cost to purify an untreated water, in the case of the standard membrane filtration although having high ability of purifying the untreated water as compared with the sand filtration, a permeate flow is extremely low (for example, 0.5 to 2 m3/m2/d), resulting in high cost required to provide a purified water.
In the meantime, the separation membrane has the following advantages as compared with the sand filtration and, therefore, if the problem associated with the high cost required to prepare the purified water due to the low permeating speed were to be successfully removed, the separation membrane appears to wide-spread as a new technique capable of superseding the sand filtration.    a. Since the permselectivity is sharp, a stable filtered liquid can be obtained without being affected by the quality of an untreated water and a safety factor is also high.    b. Complicated maintenance such as replacement of a mass of sand is little involved and the amount of materials to be disposed is minimal.    c. While the sand filtration requires coagulating and sedimenting equipments in order to improve the permselectivity, the coagulating and sedimenting equipments to be used in association with the membrane filtration can be dispensed with or may be simplified, thus making it possible to minimize the space for the system and also to simplify the processing steps.    d. Since the filtrate recovery rate is high and the drainage of the backwashing water is minimal, disposal of the backwashing water used can be simplified.
As discussed above, the reason that the permeate flow exhibited by the membrane filtration is considerably lower than that exhibited by the sand filtration appears to be because with the conventional separation membrane which is mainly used in the form of an ultrafiltration membrane or a microfiltration membrane having the particle cutoff not greater than 0.2 μm, a pure water permeate flow is originally low because of the small particle cutoff and because impurities and suspended matter contained in the untreated water are trapped almost by the separation membrane with the consequence that the pure water permeate flow is further lowered in the face of the resistance imposed by the impurities and others. In contrast thereto, the fractionating precision of the sand filtration is within the range of about 5 to 10 μm, the pure water permeate flow is originally high, and even in the presence of the impurities and suspended matter in the untreated water, they can be penetrated if the size thereof is not greater than 5 μm. Therefore, the sand filtration is less sensitive to the resistance imposed by the impurities and others and, therefore, can maintain a high permeate flow.
Although with the sand filtration impurities and others of which size is not greater than 5 μm cannot be trapped, in most of the applications the currently utilized purifying equipment in which the coagulating and sedimenting facilities and the sand filtration are utilized in combination would work satisfactorily in terms of the quality of water if the particle cutoff is within the range of about 5 to 10 μm, and it appears that the quality of water achieved by the microfiltration or the ultrafiltration region may not be necessary. Also, in applications other than the service water, the sand filtration is effective to satisfy a requirement sufficiently in terms of the quality of water and it is suspected that there are some applications in which the quality of water achieved by the microfiltration or the ultrafiltration region may not be necessary.
Of the applications other than the service water, where water for a swimming pool, public baths, spa, an aquarium, tanks for aquatic animals, a coolant used in plants, boilers, ponds and so on is desired to be purified, a circulation purification process is generally used in which portion of the impounded water is taken and, after the taken water has been subjected to a physical separating operation or a biological oxidizing treatment such as, for example, sand filtration, filtration with a filter aid, coagulation and sedimentation or pressurized floatation, the treated water is returned to the impounded water.
In recent years, the quality of industrial water and groundwater is getting worse as a result of pollution of the water source and, in view of this, it is increasingly recognized to recycle the water and to increase the safety factor. Accordingly, with the conventional physical separating technique such as, for example, the sand filtration, filtration with a filter aid, coagulation and sedimentation or pressurized floatation it often occur that the required purification level of the impounded water cannot be attained. Also, the sand filtration, the coagulation and sedimentation or the pressurized floatation has a problem in that the separation precision is not smaller than 10 μm and no suspended microparticles of a size greater than it is difficult to remove. Although the filtration with a filter aid is a process in which a pre-coating layer of the filter aid such as diatomite is formed on a filtering surface such as a filtering cloth and filtration is effected through this pre-coating layer, complicated procedures are required to introduce the filter aid into the pre-coating layer and the untreated water and to replace the filter aid and, moreover, the filter aid itself constitutes a waste to be disposed. Accordingly, the filtration with the filter aid has a problem in that disposal of the filter aid would eventually lead to environmental pollution. On the other hand, if attempt is made to purify the impounded water using a biological oxidizing process, a relatively long time is required to purify, the untreated water and a relatively large space for installation is required to compensate for it. There are other problems such as complicated maintenance, management and requirement of an apparatus for removing suspended matter after the treatment.
In the meantime, a technique in which in place of the conventional sand filtration and the filtration with the filter aid, hollow fiber membranes are used to purify the impounded water such as water for a swimming pool is well known in the art from, for example, the Japanese Laid-open Patent Publications No. 59-206091, No. 8-323396 and No. 10-121759.
The Japanese Laid-open Patent Publication No. 59-206091 discloses purification of the impounded water (pool water) with the use of hollow fiber membranes for ultrafiltration region. If the standard hollow fiber membranes having such a small separating precision are used, removal of suspended microparticles, bacteria and so on can be achieved sufficiently and a stable and high quality water can be obtained. However, according to the technique disclosed in this Japanese Laid-open Patent Publication No. 59-206091, since the filtering speed is so low that treatment of a large quantity of impounded water such as water for a swimming pool requires the use of an equipment having a large membrane area and, therefore, this known technique is impractical because of the cost incurred in preparing the equipment and the high running cost.
On the other hand, the Japanese Laid-open Patent Publications No. 8-323396 and No. 10-121759 disclose a method in which a major quantity of the impounded water is filtered by circulation through a coarse filtration such as sand filtration and only a portion thereof is purified by the use of hollow fiber membranes. However, with this method, if the proportion of the amount of filtration through the hollow fiber membranes relative to the entire amount of the impounded water is low, the water quality cannot be improved and, conversely, if the proportion of the amount of filtration through the hollow fiber membranes relative to the entire amount of the impounded water is increased, the water quality may be improved, but a problem associated with the increased cost for the equipments would occur and, therefore, no satisfactory result cannot be necessarily obtained.